"Epoxidation" of mono- and polyfunctional phenols or aromatic amines, hydantoins and triisocyanuric acid by reaction thereof with epihalohydrins (epichlorohydrin most notably) is well known. Ordinarily, the epoxidation proceeds through two successive reactions; adduction and dehydrohalogenation: ##STR1## wherein --QH.sub.x is --OH, --NH.sub.2, ##STR2## or &gt;NH, R is a non-interfering radical and the first reaction is catalyzed by an onium salt or an alkali metal hydroxide (conveniently, the same as that employed in the second reaction).
Henceforth in these specification what is said about the epoxidation of phenolic --OH groups will be intended to apply as well to --NH.sub.2 and &gt;NH groups, unless otherwise noted. Thus, the term "epoxidation product" is intended herein to mean a compound comprising a plurality of oxirane groups, at least one of which is contained in a glycidyl group derived from the dehydrohalogenation of an adduct of the oxirane ring in an epihalohydrin with an active hydrogen-containing group, such as, for example, an --OH, --NH.sub.2 or --NH-glycidyl group attached to an aromatic ring or an &gt;N-H group in which the &gt;N-- is part of an N-heterocyclic ring.
The term MEK is used herein to represent methylethylketone.
Little difficulty is ordinarily encountered in completing the foregoing second step in the epoxidation of otherwise unsubstituted monofunctional hydroxy benzenes (etc.). However, completion of dehydrohalogenation is more difficult when the phenol or amine already contains one or more glycidyl, glycidylamino or glycidyloxy groups. This is most noticeably the case when the epoxidation is carried out in a manner such that a preponderantly oligomeric product is formed (by in situ reaction of polyfunctional epoxides with unepoxidized starting phenols (etc.)). By resort to higher temperatures and caustic concentrations it is possible to push the dehydrohalogenation more nearly to completion. This technique reduces the content of "hydrolyzable chloride" in the product, but is of limited utility because it is also conducive to base-catalyzed oxirane-consuming reactions. The latter reaction results in polymerization and, when species having an oxirane functionality greater than two are present, in crosslinking; gelation of the reaction mass then may occur.
The difficulty of attaining low hydrolyzable chloride levels is greater when the structure of the starting phenol (or at least of any oligomeric halohydrin intermediate species) is inherently such as to hinder or retard dehydrohalogenation. Exemplary of such starting phenols are phenol/formaldehyde novolacs and poly(hydroxyphenyl)alkanes having functionalities of three or more.
The predominant source of hydrolyzable chloride (C-Cl groups) in glycidyl ether type epoxides is the presence of intermediate 1,2-halohydrin intermediate molecules which can be but have not been dehydrohalogenated. However, by-product species which do not have a hydroxyl group .alpha. to the C-Cl group and cannot be converted to 1,2-epoxides also constitute a source of hydrolyzable chloride. The process of the invention conveniently is referred to as a re-dehydrohalogenation step but is not limited to reactions with C-Cl species convertible to epoxides by reaction with a base.
Certain oligomeric epoxides of the latter type are prepared from 1,1,1-tri(hydroxyphenyl)alkanes. These epoxides are resins having a unique combination of properties which make them particularly suitable in curable formulations for encapsulation of electronic components which will be exposed to severe temperature and moisture conditions. (These epoxides are dislosed in U.S. Pat. No. 4,394,496, Ser. No. 316,586, filed Oct. 30, 1981.) For the latter use, the epoxide must have a low melt viscosity and a very low hydrolyzable chloride content. Because the viscosity must be held down, resort cannot be had to the conventional techniques-which may be employed to lower chloride levels at the expense of a substantial molecular weight increase-even though gelation is avoided. Thus, it would be highly desirable to find a way of modifying known epoxidation procedures to permit attainment of low chloride contents without experiencing substantially higher viscosities. To do this would be particularly valuable in the case of epoxies prepared from tri(hydroxyphenyl)methanes.
The epoxidation process claimed in the '496 patent identified earlier herein is directed to the preparation of epoxides from tri(hydroxyphenyl)methanes (in which the remaining methane hydrogen may be replaced by alkyl radicals of up to 10 carbons). However, the process is believed to be generally advantageous for the epoxidation of phenols, aromatic amines and &gt;NH groups in heterocyclic rings.
Thus, the latter process may be generally defined as the method of preparing a polyepoxide which comprises: (1) reacting a polyfunctional phenol, aromatic amine or a nitrogen heterocycle in which a reactive hydrogen is attached to each of two or more ring nitrogens, with an epihalohydrin in the presence of more than 1 and up to about 3 equivalents per --OH or &gt;N-H group of an aqueous base and essentially in the absence of coupling catalysts and solvents other than the epihalohydrin itself; (2) adding a solvent having the characteristics of a methylethylketone/toluene mixture and up to about 2 more equivalents of aqueous base; and (3) dehydrohalogenating the products of step (1) with the base present after step (2).
The following example of the foregoing process, as applied specifically to the epoxidation of 2,4',4"-trihydroxytriphenylmethane, is given in the '496 patent and constitutes the nearest prior art known of to the present applicants.
"Epichlorohydrin (114.1 grams, 1.23 moles) was added to 100 grams (0.342 moles) of 2,4',4"-trihydroxytriphenyl methane (an equivalent ratio of (1.23/(0.342.times.3)=1.2:1). The resulting mixture was heated moderately and stirred until the starting material dissolved in the epichlorohydrin. After elevating the temperature to about 90.degree. C., the rate of stirring was increased and 51.5 grams (1.29 moles) of NaOH (1.29/(0.342.times.3)=1.25 moles per phenolic hydroxyl in the starting material) was added portionwise, as a 25% aqueous solution, over a one-hour period. When addition of about 60% of the NaOH solution was completed, 100 ml (80 grams) of a 3:1 mixture of methylethylketone and toluene was added to the reaction mixture and the NaOH addition continued. Following the solvent mixture addition, the temperature of the reaction mixture decreased from about 100.degree. to about 85.degree. C. After the NaOH addition was completed, the reaction mixture was heated with stirring at 80.degree. -85.degree. C. for another 90 minutes and then mixed with another 200 ml (160 grams) portion of the solvent mixture and 50 ml of water. A concentrated brine, which formed at the bottom of the mixture, was separated therefrom. Solvent was then stripped (*) from the reaction mixture (under 20-25 inches of vacuum), with final removal of volatiles being done by steam stripping under vacuum. As a result of these operations, a clear, amber, hard and brittle resin with an Epoxy Equivalent Weight (EEW) of about 215-240, a melt viscosity of 500-1000 centistokes at 150.degree. C., and a Duran softening point of 80.degree.-85.degree. C. was obtained. Based on the EEW, the average epoxy functionality appears to be that of a dimer (theoretical EEW=216), e.g., epoxide functionality of about 4. However, from the results of GPC (Gel Permeation Chromatographic) analysis, it appears the monomer and dimer each comprise about 20-25% of the product while the trimers and tetramers together comprise about 50-60%. Various 2 gram samples of the resin were mixed with stoichiometric amounts of a curing agent (methylene dianiline) and cured for 2 hours at 90.degree. C., 4 hours at 165.degree. C. and 16 hours at 200.degree. C. Heat distortion temperatures were determined by the known TMA method and found to range from about 245.degree.-253.degree. C. Similar TMA tests on the cured tris-epoxide of leucaurin of Example 3 were found to be about 246.degree. C. FNT *Note: If the wet organic phase has not already been dried, the water content obviously will be removed in the early stages of stripping and the salt it held will precipitate. If a salt-free product is desired, stripping of course can be interrupted and the precipitate filtered out.
When the foregoing preparation is carried out in essentially the same manner otherwise but at an epi to phenolic hydroxyl ratio of 1.5, the EEW of the product drops to about 205, the monomeric epoxide content rises to about 30%, and the viscosity of the product decreases accordingly to about 400 c.s."